tank type heater, I would be better off buying a tankless 95% heater. I intend to do that in a couple of years, my old heater is only 2 years old, and I think the tankless will be better and cheaper in a couple of years. I had the gas company install a high capacity regulator and plumbed my gas with 1" pipe in anticipation of this.

I wouldn't just vent through the rim joist without a vent fan. I wasn't very clear in my OP...but my question really is about use of external vent fans like this one..

http://www.menards.com/main/water-heaters/parts-accessories/bosch-aq4-power-vent-for-horizontal-venting-of-models-1600p-1600h/p-200839.htm

I read something about this a while back. This is part of my reason for wanting to use my old heater for a couple more years in addition to tankless coming down, I suspect some of these bugs will be worked out. Are you sure you have enough gas volume for the unit? The tech from the gas company said that some people who have installed these type heaters ave experienced problems because it demands more gas. If the volume isn't high enough the heater will shut down. He said since this has been discovered he has installed many increased capacity regulators on existing meters and the home owners have had to re plumb their gas lines. He said some installers were using a reducer coupling to convert the existing 1/2" gas line to 3/4" which also choked the new heater...if you haven't already, it might be worth investigating.

Usually they're labeled "two-stage" or "multi-stage".

Believe it or not, most building codes were developed for a reason and venting gas appliances are high on the list. If you don't know what you are doing and why, please spend a few dollars and let a trained professional do this for you. If you can afford a 99% furnace, you can afford to have this done correctly. You are correct with the double wall pipe (type B gas vent) but if you don't understand the codes as to clearances from windows, the roofline, etc. you are asking for trouble.

At least take the time to study the documentation that should have come with the water heater. By the way, the cost of the correct equipment for power venting is EXPEN$IVE! It will cost you less to hire a pro to install the gas vent properly than to buy the power venting equipment.

Trust me, I spent 43 years of my life doing this stuff.

Wether you get an inspection or not is irrelevent but if you are not experienced with the installation of type B gas vent, the time and money and frustration you will encounter will not be worth what you think you are saveing.

If you need more info, pm me. I don't want to read here about a DUer who died of carbon monoxide poisening.
Bud

I certainly want it to be safe. The utility room is directly below an addition to the existing structure which we are currently building. I can install the vent through the roof and can get the code specs to be sure it is properly placed. I was trying to eliminate a hole in my roof now which would later be unnecessary. We are into this project for around $60k now and will be better able to afford an upgrade in a few years.

I appreciate your input, and the cost of doing this probably is prohibitive. Maybe I'll pony up for a heater designed to vent horizontally now, even though I feel I could save some money by waiting a few years..

Again, thanks for your expert input. I have been fortunate to find many pros like yourself both locally and online to help me with questions during this project.

Rain and weather are about as much of a problem for walls as they are for roofs. All the venting through the roof means there's better techniques and materials to go through the roof than the wall.